It is known that the RNP concept corresponds to area navigation for which are added (on board the aircraft) monitoring and alert means that make it possible to ensure that the aircraft remains in an RNP corridor around a reference trajectory. Outside that corridor there are potentially terrain features or other aircraft. The performance required for an RNP type operation is defined by an RNP value that represents the half-width (in nautical miles: NM) of the corridor around the reference trajectory within which the aircraft must remain 95% of the time during the operation. A second corridor (around the reference trajectory) with a half-width twice the RNP value is also defined. The probability of the aircraft leaving this second corridor must be below 10−7 per flight hour.
The RNP AR operation concept is even more constraining. RNP AR procedures are in fact characterized by:                RNP values which:                    are less than or equal to 0.3 NM on approach, and which may be as low as 0.1 NM; and            are strictly less than 1 NM on departure and during refuelling, and may also be as low as 0.1 NM;                        a final approach leg that may be curved; and        obstacles (mountains, traffic, etc.) that may be situated at twice the RNP value relative to the reference trajectory, whereas for the standard RNP operations an additional margin relative to the obstacles is provided.        
The air traffic authorities have defined a target level of safety (TLS) of 10−7 per flight hour. In the case of RNP AR operations, as the RNP values may be as low as 0.1 NM and obstacles may be situated at twice the RNP value of the reference trajectory, this objective is reflected in a probability of the aircraft leaving the corridor of half-width D=2·RNP that must not exceed 10−7 per flight hour.
The equipment on board an aircraft and notably the flight management unit must make it possible to achieve the target safety level if the aircraft has to use RNP AR type operations (Required Navigation Performance with Authorization Required).
The object is to have the capability to fly RNP AR procedures with RNP values up to 0.1 NM without restriction (in a normal situation and in the event of a malfunction) on departure, approach and refuelling.
Now, for an aircraft to have the capability to fly RNP AR procedures of this kind it is notably necessary to be able to eliminate from the guidance loop an erroneous source of computed guidance commands (or set points), in order to counter any effects thereof on the trajectory of the aircraft.
To be able to execute an RNP 0,1 type operation, the flight management unit must make it possible to comply with a “hazardous” type severity in the event of loss of or erroneous guidance set points. Moreover, in the event of detection of an erroneous computation, the aircraft must be able to continue to be guided automatically in order to be kept within the RNP corridor.
With a flight management unit with two flight management systems, in the event of a mismatch between the two flight management systems, the unit is not capable of identifying which of them is defective, and the aircraft can therefore no longer be guided automatically and is not in a position to execute RNP operations of this kind.